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Sansui AU-777A Strange balance behaviour
Sansui AU-777A Strange balance behaviour
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Old 8th November 2019, 09:45 AM   #1
cowneko is offline cowneko  France
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Join Date: Apr 2013
Location: Valenciennes
Default Sansui AU-777A Strange balance behaviour

Hi everyone!

I recently get my hands on a beautiful Sansui AU-777A from September 1969 in a near mint condition.
As you can imagine, after 50 years even if it was functional, the caps needed to be changed, witch I done without any trouble, thanks to a clever layout of the overall amp. I also changed all the small signal 2sc458 by hfe matched 2sc2240GR.

Everything went well and the amp sings well without any noise or hum.....but somehow during the first listening session I got the feeling that voices where not centered has I expected them to be, of course it's not a problem of phase inversion (I checked and it's definitely not the same effect). voices seems to be partially stuck to the speakers, I do not have this impression on other Sansui of the same period I already own (AU-555, Au-505), however, stereo is working fine.

I also found out that the balance pot has a weird behavior, the balance shift works only on extremes (right and left) between the extremes the balance do not change (I basically have 5 of available settings on both sides of the pot).
Seems like the two problems are linked, as if the channels where somehow partially mixed between the two extremes of the balance pot (witch explains the lack of localisation of the voices)

I tested the resistance of the pot with a multi-meter and all seems fine...
Do you happen to know where the problem may come from? Maybe I may search for a grounding issue around the balance pot, witch may put it on a floating state?

Last edited by cowneko; 8th November 2019 at 10:22 AM.
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Old 8th November 2019, 11:26 AM   #2
Max Headroom is offline Max Headroom  Australia
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Location: West Australia, near to the beach, natural ambient sounds mostly.
Quote:
I tested the resistance of the pot with a multi-meter and all seems fine...
Do you happen to know where the problem may come from? Maybe I may search for a grounding issue around the balance pot, which may put it on a floating state?

Yes, check the resistance to ground of each section of the balance control dual pot.
Service manual is easily available online.



Dan.

Last edited by Max Headroom; 8th November 2019 at 11:28 AM.
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